Need Advice Help with Circuit Design

Thread Starter


Joined Oct 15, 2008

I have a new question as a followup to my timer circuit motor control.
I am making a capacitor powered torpedo to be used in my r.c submarine, this is ALL that can fit in the small torpedo compared to the other large one.
So the capacitor discharge rate determines the run time and it is a smaller torpedo so weight is a factor.
It will use either:
1uf/2.7v electrolytic capacitor OR 10uf/3v electrolytic capacitor
Reed switch NC and open no power with a magnet near
Small DC brushed motor

Here is a basic charging circuit I found:

The Low voltage transistor: TLV2217
Would a LM393 be the same????

O.K my question is, in the above circuit, the capacitor is charged up and during the charging you can here the motor speed up until release and it goes on it`s way.

Question, Can this circuit be modified so that it charges the capacitor to a safe Max. Voltage then stops charging without blowing up?
Can I have a small LED display showing the charged voltage amount?
The main battery source for this charger is 4 -- AA batteries OR I can go to a Lipo battery as the main battery if you recommend it.
This would be portable in a little hobby box
I would also like to have a Power On / Off switch with LED light indicator

Overall I would like to keep this as Small as possible and as low cost as possible

Sorry for the big picture above.
Can you show a circuit with parts in your reply?
Thanks in advance again.

Thread Starter


Joined Oct 15, 2008
Sorry, but your explanation is a little confusing. o_O
Can you post a complete schematic of what you have?
The above drawing is ALL that I have besides a picture of the plastic box holding the above.
It has a battery supply 4.8v drops down by the Trans. through a resistor to two spring connectors which attach to each point of the capacitor for charging. Listening the the running torpedo speed is the only indicator of fully charged capacitor estimate before it blows up?


Joined Mar 19, 2019
You're not blowing up a balloon you are charging a cap from a ~5V power source regulated @ 2.5V. The cap will charge to 2.5V and stop. Theoretically, it will never stop charging as it will never reach the same voltage as the regulator output but you probably couldn't measure the difference as it is increasingly infinitely small. You could use a cap rated higher than the power supply voltage and not need a regulator or use an LM317 regulator adjusted for whatever max output voltage you require. It all depends on what your output voltage needs to be. A circuit diagram is needed as my crystal ball is very cloudy without more details.


Joined Sep 17, 2013
Unfortunately, neither capacitor will hold anywhere near enough charge to power the motor for more than a few milliseconds :(. Even if you used a 1000uF cap charged to 3V, the charge would be 3V x 1000uF = 3 milliCoulombs. That could provide, theoretically, 1mA for 3 sec or 100mA (a likely start-up current for a small DC motor) for 30mS.
Instead of a DC motor drive, have you considered using a twisted rubber-band drive?


Joined Aug 1, 2013
Because you mention a 2.7 V cap, is the 1 uF value a typo? A 1 uF cap won't power anything, but a 1 F cap will. Are you using super/ultra capacitors?

What are "charging plates"?


Thread Starter


Joined Oct 15, 2008
Hi Sorry
Yes I meant to type F not uF for both cap. values. Charge plates according to the drawing are just 2 plates/pins, one +ve and one -neg. to connect charging connectors to.

Trouble when you have hand tremors, Sorry.

Thanks again to ALL for the help.